Microphones generate electrical signals representative of noises and sounds in the environment around the microphone. Microphones are important devices for many electronic devices because sound, and in particular speech, is one of the most important manners of interaction between a human and an electronic device and a human to another human through an electronic device. Microphones generally produce analog signals, but processors within electronic devices are generally digital components that operate on digital signals. Thus, the analog signals of the microphones must be converted to digital signals for further processing within an electronic device. For example, the analog microphone output may be converted to a digital signal to allow an individual's speech to be transmitted from one cellular phone to another cellular phone. In another example, the analog microphone output may be converted to a digital signal to allow a cellular phone to detect speech commands from a user. The component coupled to the microphone for converting the analog signal to a digital signal is an analog-to-digital converter (ADC).
ADCs are thus important components in electronic devices. One complication with the use of ADCs is that the coupling configuration between the microphone and the ADC changes how the ADC processes the analog output of the microphone to generate a digital representation of the microphone output. That is, an ADC must be matched with the particular microphone coupled to the ADC. This restriction inhibits the ability of a user to use any microphone with their electronic devices. Further, this restriction inhibits the ability of a manufacturer to substitute different microphones due to supply shortages. Some different coupling configurations are shown in FIGS. 1A-1D.
Microphones are either fully-differential (FD) or pseudo-differential (PD) and either AC-coupled or DC-coupled into an analog-to-digital converter (ADC). Thus, there are at least four different microphone topology configurations requiring different operations from and interfaces with an ADC. FIG. 1A illustrates an AC-coupled fully-differential configuration for a microphone and ADC. A microphone 102 may provide outputs 104 and 106. The outputs 104 and 106 are also the inputs to ADC 108, which generates a Dout digital signal containing a digital representation of sounds captured by the microphone 102. In AC-coupled configurations, such as FIG. 1A, capacitors 112 and 114 are coupled between the microphone 102 and the ADC 108. The capacitors 112 and 114, along with input impedance of the ADC 108, create a high-pass filter to block DC signals from the microphone 102 from reaching the ADC 108. The capacitors 112 and 114 may be either integrated into a chip along with the ADC 108 or separate from a chip containing the ADC 108. In either case, the capacitors 112 and 114 consume space in an electronic device that increases the dimensions and thickness of the electronic device. Similar to FIG. 1A, FIG. 1B illustrates an AC-coupled pseudo-differential configuration for a microphone and ADC. The pseudo-differential configuration 120 of FIG. 1B is similar to the fully-differential configuration 110 of FIG. 1A, but with one terminal of the microphone 102 grounded to node 116.
Alternatively to the AC-coupled topologies of FIGS. 1A and 1B, DC-coupled topologies may be implemented to interfacing an ADC with a microphone. DC-coupled microphone topologies do not require capacitors 112 and 114 to block the DC value of the microphone outputs. Eliminating the capacitors reduces cost and size, but requires extra processing to make the ADC compatible with fully-differential (FD) and pseudo-differential (PD) microphones. FIG. 1C and FIG. 1D illustrate a DC-coupled fully-differential (FD) configuration 130 and a DC-coupled pseudo-differential (PD) configuration 140, respectively. One example of the extra processing is that a fully-differential (FD) microphone 102 may provide output values of Vin and Vip, but these values may be mismatched from each other and also from the desired DC value for correct operation of the ADC. Another example of a configuration requiring additional processing is that of a pseudo-differential (PD) microphone in which the Vin signal is connected to ground 116. In both of these examples, the ADC 108 must apply processing specific to the microphone configuration of either FIG. 1C or FIG. 1D.
As described above, each of the four configurations of microphone topology shown in FIG. 1A, FIG. 1B, FIG. 1C, and FIG. 1D require different operation and interfacing with an ADC. For example, AC-coupled microphones require a capacitor at the input of the ADC to block DC signals. As another example, AC-coupled microphones require a common mode voltage generator coupled to the ADC to set the DC values of the inputs Vin and Vip. As yet another example, a DC-coupled fully-differential microphone requires processing by an ADC to match the microphone input signals to a desired DC value. Because of these different requirements, an ADC is conventionally designed to match a specific microphone configuration and is then generally not usable for other microphone configurations.
Further, undesired effects may occur when a microphone is coupled through a differential input to interface with the ADC. For example, the common mode (CM) voltage value of fully-differential inputs may not be matched, such that the common mode at input node 104 is different from the common mode at input node 106. Any mismatch between the input CM values may translate into a differential signal that can clip and saturate components the ADC. As another example, an AC signal amplitude mismatch between the differential inputs may produce similar clipping and saturation in the ADC. FIGS. 1E-1G illustrate examples of these undesired effects. The graphs of FIGS. 1E-1G illustrate differential input signals along with a modulator output when input voltages are matched in FIG. 1E, have mismatched CM voltages in FIG. 1F, and have mismatched DM voltages in FIG. 1G. Both the mismatched CM and mismatched DM examples of FIG. 1F and FIG. 1G, respectively, may result in quantizer saturation or clipping within an ADC, and thus poor ADC performance. In the mismatched CM of FIG. 1F, the modulator output experiences a DC shift 132 approximately equal to half of the input CM mismatch value (ΔVCM/2). In the AC amplitude mismatch of FIG. 1G, there is no offset shift on the output code, but the mismatch causes a gain scaling that may result in a symmetric clipping in the quantizer.
Although undesired effects during fully-differential (FD) operation are described above, undesired effects may also occur during pseudo-differential (PD) operation in which one input is coupled to ground and the other input produces a signal Vip=Vcmi+Vdm. In pseudo-differential (PD) operation, an unbalanced DC shift between differential outputs of components in the ADC may cause swing-driven distortion, such as caused by even harmonics. Such undesired effects also result in poor ADC performance.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved electrical components, particularly for ADCs employed in consumer-level devices, such as mobile phones. Embodiments described herein address certain shortcomings but not necessarily each and every one described here or known in the art.